Police with shields and clubs pushed back a small group of protesters who tried to head toward the security fence around the site of the G-20 summit.

Some demonstrators hurled bottles at police.

About an hour later the group, dressed all in black, smashed the windows of a bank, a coffee shop and some stores.

Police maintained tight security around the summit site, but largely did not intervene as marchers destroyed property.

"We have an enormous amount of resources at our hands," Constable Wendy Drummond who characterised the police response as "measured" said.

The dynamic in the crowd changed around 3 pm as police donned gas masks.

Some parts of the riot line are as many as three officers deep as the crowd chanted: "Let us through!" Several protesters were being treated for injuries by fellow demonstrators amid reports that paramedics faced delays in getting to the area.